arXiv subjects

Michael Seiler

Publications and source records attributed to Michael Seiler.

3 recordsLinked to original sources

arXiv2019

The radial density profile of Saturn's A ring

In this work, we model the radial density profile of the outer A ring of Saturn observed with Cassini cameras (Tiscareno & Harris 2018): An axisymmetric diffusion model has been developed, accounting for the outward viscous flow of the ring material, and the counteracting inward drift caused by resonances of the planet's large outer moons. It has been generally accepted that the 7:6 resonance of Janus alone confines the outer A ring which, however, was disproved by Tajeddine et al. (2017). We show that the step-like density profile of the outer A ring is predominantly defined by the discrete first-order resonances of Janus, the 5:3 second-order resonance of Mimas, and the overlapping resonances of Prometheus and Pandora.

astro-ph.EP↗

arXiv2018

Analyzing Bleriot's propeller gaps in Cassini NAC images

Among the great discoveries of the Cassini mission are the propeller-shaped structures created by small moonlets embedded in Saturn's dense rings. We analyze images of the sunlit side of Saturn's outer A ring, which show the propeller Bleriot with clearly visible partial propeller gaps. By determining radial brightness profiles at different azimuthal locations, we obtain the evolution of the gap minimum downstream of the moonlet. From the radial separation of the partial propeller gaps we estimate the Hill radius of Bleriot to be about 400m. Further, we fit the analytic solution from Sremcevic et al. (2002) describing the azimuthal evolution of the surface mass density in the propeller gap region to the azimuthal gap evolution obtained from Cassini images. From these fits, we estimate a kinematic shear viscosity in the range of 60 cm$^2$/s to 100 cm$^2$/s in Bleriot's ring region. These values are consistent with the parametrization given by Daisaka et al. (2001) and agree well with values estimated for the Encke gap edge (Tajeddine et al. 2017; Graetz et al. 2018).

astro-ph.EP↗

arXiv2017

Hydrodynamic simulations of moonlet induced propellers in Saturn's rings: Application to Bleriot

One of the biggest successes of the Cassini mission is the detection of small moons (moonlets) embedded in Saturn's rings which cause S-shaped density structures in their close vicinity, called propellers (Spahn and Sremcevic 2000; Tiscareno et al. 2006; Sremcevic et al. 2007). Here, we present isothermal hydrodynamic simulations of moonlet-induced propellers in Saturn's A ring which denote a further development of the original model (Spahn and Sremcevic 2000). We find excellent agreement between these new hydrodynamic and corresponding N-body simulations. Furthermore, the hydrodynamic simulations confirm the predicted scaling laws (Spahn and Sremcevic 2000) and the analytical solution for the density in the propeller gaps (Sremcevic et al. 2002). Finally, this mean field approach allows us to simulate the pattern of the giant propeller Bleriot, which is too large to be modeled by direct N-body simulations. Our results are compared to two stellar occultation observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS), that intersect the propeller Bleriot. Best fits to the UVIS optical depth profiles are achieved for a Hill radius of 590 m, which implies a moonlet diameter of about 860 m. Furthermore, the model favours a kinematic shear viscosity of the surrounding ring material of $\nu_0 = 340$ cm^2/s, a dispersion velocity in the range of 0.3 cm/s $< c_0 <$ 1.5 cm/s, and a fairly high bulk viscosity $7 < \xi_0/\nu_0 < 17$. These large transport values might be overestimated by our isothermal ring model and should be reviewed by an extended model including thermal fluctuations.

astro-ph.EP↗